The present invention relates to a ceramic matrix composite used for gas turbine members and the like and a method of manufacturing the composite, especially to a composite having a ceramic matrix formed by reaction sintering and ceramic fibers having a coat layer such as a sliding layer combined in the matrix.
In general, ceramic sintered bodies are used in wide fields such as components of industrial electric equipment, airplane members, automobile members, electronic equipment, precision machine members, electronic materials like semiconductor materials and structural materials because the they show little decrease in their strength up to a high temperature and have excellent properties in hardness, electric insulation, abrasion resistance, corrosion resistance and light weight compared with metallic materials.
However, the tensile stress of the ceramic sintered body is so weak compared with its compressive stress that it has a inherent drawback that, when there is a potential defect portion, a fracture develops in one moment under a tensile stress--so called brittleness--because the stress is focused on the defect portion. Actually, ceramic members are easily fractured by a collision of foreign materials, inhibiting its practical application for, for example, gas turbine members.
Therefore, it is strongly desired that the ceramic sintered bodies are endow with high tenacity or their fracture energy is increased for applying it to structural members of ceramics such as gas turbine members, air plane members and automobile members for which a high reliability as well as heat resistance and strength at a high temperature are require.
Under these requirements, ceramic matrix composite formed by dispersing and compounding composite materials such as fibers, whiskers, plates and grains comprising inorganic materials or metals in the matrix have been noticed as ceramic sintered bodies improved in fracture tenacity, fracture energy level and heat shock resistance, facilitating research and development of these materials for practical applications in many research institutes around the world. Especially, when fibers such are compounded into a ceramic matrix composite, their effects for improving fracture resistance become so large that their practical applications are expected.
Among these ceramic matrix composites, a composite material as a high temperature material having silicon carbide (SiC) in its matrix has been especially noticed. While examples of methods for synthesizing the SiC matrix in the composite material include, taking heat resistance of fibers into account, CVI (Chemical Vapor Infiltration) method, PC (precursor) method and a reaction sintering method, the reaction sintering method is noticed as a representative example by which an initial fracture strength can be so easily improved that a high reliability is obtained since a SiC matrix with a compact structure is formed by the method.
In the ceramic matrix composite described above, it is an important problem to adequately control the bonding at the boundary face between the matrix and fibers in order to display the composite effect of fibers, because the composite effects like bridging and pulling out are not fully displayed when fibers and the ceramics are tightly bonded, being liable to be fractured due to brittleness. As a countermeasure, a method for coating a sliding layer comprising boron nitride (BN) and the like is known in the art for allowing the fiber to slide against the matrix by weakening the bonding between the fiber and matrix.
However, the sliding layer described above had a problem that it was denatured, decomposed or disappeared by a reaction with the materials forming the matrix when a reaction sintering method was applied. When a reaction sintering method in which a SiC matrix is formed by impregnating a preform formed with ceramic fibers with molten Si is applied, the reactivity of the molten Si to be impregnated is so high that it readily reacts with the sliding layer or fiber itself, thereby sometimes arising a problem that the composite effect of fibers described above can not be fully exhibited.
As one of known method of suppressing the above reaction of the sliding layer with molten Si during the reaction sintering, there is a recently proposed ceramic matrix composite including not only the sliding layer but also a reaction suppressing layer such as a SiC layer covering the sliding layer.
The proposed ceramic matrix composite comprises a ceramic matrix having SiC formed by the reaction sintering as a main phase and ceramic fibers compounded in the matrix, wherein a BN layer as the sliding layer capable of inducing sliding of fibers against the matrix, and a SiC layer, which is a reaction suppressing layer suppressing a reaction of the BN layer with molten Si, are disposed between the matrix and fibers as a boundary layer.
The method for producing this ceramic matrix composite is described below. First, a BN layer is formed on the surface of fibers comprising SiC using CVD method, followed by forming a SiC layer on this BN layer using CVD method. Next, the fibers are bundled to form a bundle of fibers, which is formed into a fiber structure unit by braiding.
Then, a ceramic powder material is filled in the gaps among fibers in this fiber structure unit and in the vicinity of the fibers by a slip cast method followed by drying, thereby forming a molded body including fibers. This molded body is heated at 1420 to 1500.degree. C. so that the molded body is impregnated with molten Si for allowing the C component in the molded body to react with molten Si, thereby compounding the fibers in the matrix containing SiC as a main phase. Thus, a ceramic matrix composite in which BN layer and SiC layer exist in the interface between both of the matrix and fibers can be obtained.
However, the ceramic matrix composite proposed in the example above had problems that cracks are so liable to generate in the SiC layer during braiding that molten Si invades into the BN layer through cracked portions in the SiC layer and reacts with the BN layer during the reaction sintering, that is, a part or, at worst, almost all of the BN layer disappears without exhibiting the effect of the SiC layer as a BN protective layer, thereby making it impossible to make a boundary layer of desired design to exist between the matrix obtained and the fibers.
Countermeasures for the above problems are proposed comprising 1) a method for producing a fiber structure unit by braiding in a state where a BN layer is formed on the surface of fibers, followed by forming SiC in this fiber structure unit by CVD method or CIV (Chemical Vapor Infiltration) method, and 2) a method for previously dissolving B into a solid Si before preparing molten Si.
According to method 1) above, such problems are supposed to arise that the difference in film thickness between the vicinity of the surface of this fiber structure unit and the interior of it becomes large by coating the fiber structure unit with SiC, and coating inside of the fiber structure unit is technically difficult. Therefore, this method is not always effective since much labor and time are required for overcoming the problems.
According to method 2) above, the phenomenon that B in the BN layer is dissolved into molten Si can be prevented to some degree when B is previously dissolved into a solid Si before preparing molten Si up to a solubility limit of B at the sintering temperature.
However, since SiC is formed during reaction sintering by a reaction of molten Si with C, there would be another problem that the apparent concentration of B in molten Si becomes so high that a quantity of B exceeding its solubility limit in the solid Si precipitates as silicon borate. The precipitated silicon borate blocks the pathway for impregnating with molten Si to prevent the reaction sintering.
As a countermeasure of this problem, it can be worked out that the quantity of B dissolved into solid Si before preparing molten Si may be previously lessened. However, this is also not always advantageous because controlling the amount of B dissolved into solid Si before preparing molten Si is difficult since, in this case, B in the BN layer is decomposed and dissolved in molten Si.